This invention relates to a process for introducing platinum atoms into a semiconductor body to reduce the minority lifetime of carriers within the body, and more particularly relates to a novel process which avoids a silicon dioxide barrier to the diffusion of platinum atoms into a silicon wafer.
It is frequently desirable to reduce minority carrier lifetime in semiconductor wafers, die or bodies, which terms are used interchangeably herein. Gold is often used as a lifetime killer but gold tends to increase the leakage current of semiconductor devices at high temperatures. It is also well known to irradiate wafers to reduce minority carrier lifetime, but it has been found that lifetime control induced by radiation is not stable and may anneal out over time and during the die attach process.
Platinum atoms dispersed throughout a semiconductor wafer would provide an extremely effective control of lifetime of minority carriers, will not anneal out as is the case for radiation-treated wafers and does not cause high hot leakage current as is the case with gold. Thus, for the same die, the same silicon specification and for roughly equivalent lifetime reduction, hot leakage currents of 20 microamperes, 400 microamperes and 3500 microamperes are obtained for platinum, electron irradiation and gold, respectively.
The use of platinum as a lifetime killer is disclosed in U.S. Pat. No. 3,640,783. This patent discloses that a silicon wafer can be heated to a temperature adequate to cause platinum to diffuse throughout the body of a silicon wafer but insufficient to cause substantial junction movement. Platinum to kill lifetime is also disclosed in U.S. Pat. No. 3,963,523. It is also known that platinum can be sputter-deposited or co-evaporated with tungsten silicide as disclosed in U.S. Pat. No. 4,322,453 where, however, platinum is not employed as a lifetime killer but is simply intended to increase the conductivity of a tungsten silicide layer. Another disclosure of the desirability of platinum as a lifetime killer is contained in U.S. Pat. No. 4,777,149 in which platinum is applied directly to the surface of a silicon body.
In carrying out processes in which platinum is to be diffused into a silicon wafer, it has been found that a very thin layer of oxide, even one a few atoms thick, will block effective platinum diffusion into a silicon body. It is believed that this problem has prevented the use of platinum as a lifetime killer in commercial products.
The present invention provides a novel method whereby platinum can be diffused into a silicon body which avoids the possibility of the formation of a blocking silicon dioxide layer.